Three sigma process capability

However, this kind of reliability is not enough when one considers that there are many processes required to produce a product. Even a three sigma process capability will produce 3000 defects in a million tries. With many processes required to make a product, at three sigma levels there will be many defective products. A six sigma capability, on the other hand, means that only 3.4 outcomes in a million attempts are out of specification. 

The Six Sigma process can be used to benchmark the capability of a validation process and hence indicate the significance of any opportunity for improvement. Average capability is characterized by a Three Sigma performance. Six Sigma indicates a world class performance anything beyond Six Sigma is not considered cost-effective. ... 

The quality performance of a stream from a distillation column can be evaluated by the average impurity concentration in the stream and the distance from the nearest specification limit (DNS) compared to the standard deviation (Equation 7.1). The value of DNS is three times the process capability index, Cpk. The DNS value is easy to communicate in the number of sigma units ... 

Thirty years ago the industry controlled the molding process to 3 sigma limits. Today the industry strives to control to 6 sigma limits to produce fewer than three defective parts in a million. How close one is toward that goal is determined by calculating the CP (process potential) and CPk (process capability). CP calculates the ratio of the process specifications against the estimated process variation as shown by the following equation. 

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